There are numerous of ways of designing a housekeeping supply for isolated switched mode power supplies (SMPS:es). The largest problem is to supply the isolated secondary side with reliable power. Fly-back solutions exist, but the high voltage isolation requirement of basic isolation and high isolation voltage cause huge problems when designing the device, see e.g. the publication US 2011/0090724 A1. In order to supply the primary side with regulated power the transformer requires typically three windings.
An alternative approach is to use an auxiliary winding in the main transformer, or tap off power from the main transformer. This requires that the main switches are switching. These cause problems, during pre-bias starts and during fault handling when the output must be turned off, while the housekeeping power at the secondary side is still necessary.
The standard solution for signaling over the isolation barrier is to use optocouplers or signal transformers. Both these components take up valuable board space and have their drawbacks in terms of manufacturability, reliability, and aging.